In conventional natural gas conditioning, natural gas, having passed through an acid gas removal unit (AGRU) and dewpoint control, is often dehydrated by passing the natural gas through a system of vessels or units referred to as a dehydration unit including adsorption beds made up of molecular sieve particulate material, also referred to as mole sieve. Such a system includes at least two vessels in which one of the vessels contains saturated molecular sieve that is in regeneration mode, while the other one or more vessels are operated in dehydration or adsorption mode. During dehydration mode, water is adsorbed onto the molecular sieve material; and during regeneration mode, water is desorbed from the molecular sieve. Typically, the regeneration is effected by passing hot dry natural gas, i.e., natural gas having been dehydrated, over the saturated molecular sieve. This requires a large compressor to return hot dry natural gas to a location upstream of the AGRU upstream of the dehydration unit.
Dehydration of natural gas is typically accomplished by flowing the gas over zeolite-based molecular sieve adsorbent. Water in the gas is preferentially adsorbed by the molecular sieve. Removal of water from the gas using molecular sieve dehydration is a vital process component in any liquefied natural gas (LNG) plant to meet moisture content specifications (down to 0.1 ppmv). Natural gas can contain additional contaminants such as hydrogen sulfide, mercaptans, carbonyl sulfide, etc. that are partially co-adsorbed by the molecular sieve. During high pressure regeneration, system design problems can result in water and hydrocarbon refluxing, poor water desorption, and high residual water content within the molecular sieve after regeneration. This can result in early moisture breakthrough and economic losses associated with frequent molecular sieve change outs and low dehydrator availability
During regeneration, a regeneration gas can be used to heat the molecular sieve bed to desorb water. If the molecular sieve bed is regenerated at high temperature and low pressure, then the regeneration gas may be a slip stream of dry gas, storage tank boil off gas, or any other suitable dry gases. If the regeneration is conducted at high pressure and large vessel diameters, then the vessel thickness and choice of materials will create additional heat load on the regeneration system. In addition, the high operating regeneration pressure can result in water and hydrocarbon refluxing and lower desorption rate and efficiency.
The regeneration gas may contain contaminants such as oxygen that reacts with hydrogen, hydrogen sulfide and/or hydrocarbon (e.g. propane) at high regeneration temperatures resulting in the formation of unwanted by-products such as sulfur, sulfur di-oxides, water and carbon dioxide. These by-products can build up in downstream units, or in the fuel system causing problems such as fouling, and off-specification products. Furthermore, the complete regeneration of molecular sieves is not achieved because of the contaminants present resulting in sub-optimal performance of the dehydration unit. This may also be accompanied by damage caused to the molecular sieve resulting in reduced operating life. One known solution is further purification of the regeneration gas by using additional adsorbents. However, such schemes are expensive and will not always result in full contaminant removal of the regeneration gas.
Referring to FIG. 1, dehydration of a gas such as natural gas feed stream 1 is typically done by flowing a wet gas 23 over a bed of zeolite-based molecular sieve adsorbent material (not shown) in a vessel 2A. As a result the molecular sieve adsorbent material becomes saturated with water and must be regenerated after a period of use. The adsorbent is regenerated in vessel 2R at high temperature by flowing dry regeneration gas 3, which is typically a slip stream of dried process gas 4, over the bed of molecular sieve adsorbent material. The regeneration gas is then cooled in a condenser 5, free water 6 is separated in a separator 11 and removed, and the remaining gas 7 is compressed by a compressor 8 and returned through line 46 to the front-end of the plant, upstream of the acid gas removal unit 9 which also receives the feed gas 1. The quantity of regeneration gas 3 available is limited by the capacity of the recycle compressor 8, regeneration gas heater 10, regeneration gas cooler (also referred to as condenser) 5 and the capacity of the front-end equipment in the system including the acid gas removal unit 9. If there are any problems with the dehydration unit not being able to meet dried gas specifications, very little can be done to improve the regeneration of the molecular sieves due to these overall system constraints.
There exists a need for a more robust, more flexible and less costly method and system for regenerating saturated molecular sieve in a gas dehydration unit, particularly which increase the availability of regeneration gas.